1. Field of the Invention
The present invention relates to an ink jet print head having an actuator which consists of a longitudinal vibration mode piezoelectric vibrator.
2. Related Art
A conventional on-demand type ink-jet recording head comprises a plurality of pressure generating chambers for generating ink pressure by means of piezo-electric vibrators and heating elements. A common ink chamber supplies ink to each of the pressure generating chambers via a flow channel for each respective pressure generating chamber. Each pressure generating chamber communicates with a nozzle so that the ink-jet recording head can send a stream of ink drops from each nozzle to a recording medium in accordance with a driving signal to the pressure generating chambers. The driving signal corresponds to a print signal.
In a first conventional ink-jet recording head, a resistive wire for generating Joule heat is provided in the pressure generating chamber as the pressure generating means for causing ink drops to be discharged through the nozzle. This conventional device makes use of bubble generating pressure and is known as a bubble-jet type print device.
On the other hand, a high speed drive actuator for an ink jet print head expands and compresses a pressure generating chamber to suck in ink and to form ink droplets. The actuator is constructed with a piezoelectric vibrator having a longitudinal vibration mode, which is expandable in its axial direction and has a structure consisting of piezoelectric sheet-like members and conductive sheet-like members, alternately layered one on another. A part of the pressure generating chamber is formed with an elastic plate, and the chamber communicates with a nozzle hole associated therewith.
Although the bubble-jet type head makes it feasible to readily provide an inexpensive, high-density apparatus, the heat generation causes the deterioration of ink and the 1s head itself. By contrast, the piezoelectric type features no ink deterioration because heat generation is not a factor. Therefore, a wide range of inks may be used, and lower operating costs result because the life of the head is semipermanent. Moreover, the high-speed driving of the piezo-electric vibrator having the vertical vibration mode, and the alternate repetition of the expansion and contraction of the pressure generating chamber by bringing the piezo-electric vibrator into contact with the pressure generating chamber allows the piezo-electric type print head provide higher speed printing than the bubble-jet type.
Further, when the longitudinal vibration mode piezoelectric vibrator is compared with a piezoelectric vibrator of the type in which the surface thereof is bent for vibration, the former has a smaller contact area where it contacts with the pressure generating chamber than the latter, and may be driven at higher speed than the latter. Accordingly, the former is capable of performing the printing operation at a higher speed and also at higher resolution. Therefore, while both types of piezo-electric vibration modes may be used, the longitudinal type is preferable.
While the longitudinal vibration mode piezoelectric vibrator may be driven at high speed, the attenuation rate of the residual vibration is small. This is because fluctuations in pressure remain in the chamber even after the pressure is generated in the chamber to discharge ink drops. After discharge, an ink meniscus within the nozzle recovers toward the tip of the nozzle at a resonance period (Helmholtz resonance period) specific to the materials and dimensions of pressure generating chamber. As a result, a large vibration is left after an ink droplet is shot forth.
Because the residual vibration affects the behavior of the meniscus, the position of the meniscus is indefinite when the next ink droplet is to be jet out. This may be explained by the fact that the period of the residual vibration is minute and shorter than the time required for the meniscus to reach the tip of the nozzle (the time is hereinafter called the "recovery time" of the meniscus). When high-frequency driving is carried out, the discharged ink drops may become unstable because the meniscus is insecure if ink drops are caused to be discharged before the minute residual vibration is sufficiently settled. Consequently, the direction in which the ink drops are jetted from the nozzle varies, and ink misting occurs when the meniscus overshoots the nozzle. The result is deterioration of the print quality. This hampers improvements in the response frequency of the ink jet recording head.
The vibration behavior of the meniscus varies not only with dimensional variations in the flow channel but also varies with the physical properties of material and ink. The environmental temperature makes the meniscus behavior vary further. Thus, the residual vibration of the meniscus cannot effectively controlled by a fixed driving method. Because many variables must be considered, the production cost increases. In addition, freedom in design is reduced because the dimensions of the flow channel need severe control, and less latitude is allowed in selecting material for use in forming the flow channel and for ink selection.
In addition, there arises the following problems. When the pressure generating chamber is expanded, the meniscus within the nozzle is drawn to the pressure generating chamber side. However, the meniscus is gradually recovered toward the tip of the nozzle as ink is gradually supplied into the pressure generating chamber. The discharge speed of ink drops is made constant by causing ink to be discharged after the meniscus reaches the tip of the nozzle, irrespective of the discharge timing. When the high-frequency driving is carried out, however, the ink has to be discharged before the meniscus thus drawn satisfactorily reaches the tip of the nozzle, depending on the recovery time of the meniscus since the expansion and contraction of the pressure chamber need to be carried out at short lead time.
Moreover, it is preferred to have the ink discharged in such a state that the meniscus has been drawn in to a certain degree in order to secure the discharge speed of ink drops and a stable discharge of ink.
The drawn quantity of the meniscus and the recovery time up to the tip of the nozzle vary with the dimensions of the flow channel and the physical properties of material and ink, similar to the meniscus vibration after the ink is discharged. Consequently, the method of causing ink to be discharged at fixed timing produces variation in the drawing position of the meniscus at the time of discharging ink. This varies the discharge speed of ink drops and the discharge quantity of ink. As set forth above, to maintain consistent print quality taking into account these factors, the production cost increases, whereas freedom of design is reduced because the dimensions of the flow channel need severe control, and less latitude is allowed in selecting material for use in forming the flow channel and also for ink selection.